1. Field of the Invention
The present invention relates to an optical wavelength converting device comprising a waveguide of core made of a nonlinear optical crystal and a clad surrounding the core for converting a wavelength of a fundamental wave entering the core to a 1/2 wavelength of a secondary harmonic, which utilizes the so-called Secondary Harmonic Generation.
2. Description of the related art
There are an optical wavelength converting element having a waveguide made of a secondary nonlinear optical crystal with a periodic domain inversion structure, which generates the secondary harmonic by using quasi-phase matching (QPM) when a fundamental wave is injected and passes though the waveguide. Such an optical wavelength converting element using the QPM and SHG is so called a QPM-SHG element. The periodic domain inversion structure of the QPM-SHG element is a domain-inverted grating in which the polarization direction of the waveguide is periodically and alternately inverted along the extending direction per coherence length. The waveguide of the QPM-SHG element has an output property that as this secondary harmonic output propagates, it periodically reaches the peak and trough levels every coherence length during the propagation of the fundamental wave. In this way, the QPM-SHG element adds the outputs to increase the total secondary harmonic output, by utilizing the domain-inverted grating portion in the waveguide.
Generally, a QPM-SHG element comprising a LiTaO.sub.3 substrate has a high optical conversion efficiency but has a considerably small degree of permitted limit for the fundamental wavelength injected therein. Therefore, when only using such a QPM-SHG element and a semiconductor laser emitting a fundamental wave the wavelength of which fluctuates caused by a driving current or temperature, it is difficult to achieve a stable wavelength conversion with a high efficiency.
There are a spectral feedback intra-cavity method for feed-backing a fundamental wave emitted from a semiconductor laser and passing a QPM-SHG element to that semiconductor laser to stabilize the stimulated wavelength of fundamental wave. As shown in FIG. 1, a semiconductor laser 1 is optically coupled through a coupling optical system 2 to one end of a waveguide 4 with a domain-inversion in a QPM-SHG element 3, and the secondary harmonic and fundamental wave emitted from the other end of the waveguide through a coupling optical system 5 are separated by a dichroic mirror 6. Only the separated fundamental wave is reflected by an external grating 7 to be feed-backed through the waveguide 4 to the semiconductor laser 1. In this way, the wavelength tuning of semiconductor laser is achieved by the feedback of fundamental wave to the waveguide with rotation angle of the external grating so that the emission of laser beam is stabilized.
A semiconductor laser inherently has many stimulated modes. One of the modes may be mode-locked by injecting a laser beam from the out side to the semiconductor laser so that a single-mode light is stimulated by the external light. Namely, induced emission occurs in the semiconductor laser in a such a manner that light having the same wavelength of light entering a laser medium from the out side is gradually amplified and stimulated. Therefore, the stimulated light is stabilized by feedback of light of a constant wavelength from the external grating.
By using the external feedback of the backward light of a single wavelength i.e., the spectral feedback intra-cavity method, the emission of semiconductor laser is stabilized. Not only the QPM-SHG element but also other SHG elements are utilized as far as a light beam of fundamental wave is injected to the end surface of waveguide of the optical wavelength converting element, so that the secondary harmonic having a 1/2 wavelength as great as that of the fundamental wave is obtained.
Although there is a demand for high stabilities both of a stimulated wavelength of a fundamental wave and an output level of the semiconductor laser as a light source to be used in an optical wavelength converting mechanism, a waver of the stimulated wavelength including a low frequency through a high frequency components causes a noise in the output of the optical wavelength converting output.
In the conventional optical wavelength converting mechanism, the fundamental wave of semiconductor laser is insufficient to be stabilized in its wavelength and output resulting in a lack of output stability of the secondary harmonic with a high noise. In addition, the applied electric current to the semiconductor laser is restricted, so that a high-output optical wavelength converting device becomes unusable or inoperative.